Airborne Laser Terrain Mapping (ALTM) systems use a Time-of-Flight (TOF) LiDAR to measure the distance from a system mounted in an aircraft, to the ground beneath the aircraft. A short pulse of visible or infra-red light is emitted by a light source such as a laser, and directed towards a target. The light pulse propagates to the target and a fraction is reflected and travels back to the LiDAR system where it is detected by a high-speed optical detector such as an avalanche photodiode, which converts the light pulse to an electrical signal which is then amplified. By measuring the time interval from the instant the light pulse was emitted to when the return signal was received, the distance to the target can be calculated using the accurately-known speed of propagation of the light pulse. The TOF can be measured by an electronic subsystem such as a Time Interval Meter, by digitizing the echo received and analyzing the waveform, or other means.
When the laser is fired, there is a very brief period when the detector might see some scattered light. This could be caused by reflections from internal optical components, a window at the output of the system, a window in the aircraft through which the system operates or backscattering from the first few meters of the air below the aircraft. If the echo from a previously-emitted laser pulse were to arrive at the detector during this brief period, it would not be distinguishable from the scattered light pulse and if the scattered light produced a signal of much higher amplitude than the return pulse from the target, it would swamp the echo and render the system blinded for a period of time. The pulse from the unwanted scattered light causes a blind zone during which the system is not able to respond to the return signal and measure the TOF. Consequently no range data can be computed and essentially the laser shot is wasted. Currently, all existing airborne laser mapping systems have this limitation.
For an ALTM operating at a high pulse repetition frequency (PRF), the range to the target could be such that the TOF is many times the time interval between two successive firings of the laser. Firing the laser before the return pulse from the target is received results in more than one pulse in the air at the same time. If for example the target range and laser PRF were such that there were five pulses in the air at the same time, there could be five blind zones which would significantly increase the possibility of the echo being masked and reducing the possibility of obtaining a valid range measurement. Planning the flight altitude to minimize the impact of blind zones is virtually impossible at high laser PRFs because the TOF changes with aircraft height above ground, the scanner excursion angle, the aircraft roll, pitch or heading as well as the topography of the terrain itself.